In the period of about 1940 to 1950, snorklers and spear fishermen began to use swim fins that were nothing more than paddle-like extensions of the feet of the user, providing more area for presentation to the water surface. Thus, a more efficient coupling was established between the swimmer's musculoskeletal structure and the water. Propulsion, speed and maneuverability were enhanced. See, for example, U.S. Pat. No. Re. 23,006 to Churchill, of June 15, 1948. In the early 1950s, Giovanni Cressi and Luigi Ferraro of Genoa, Italy recognized that efficient coupling depended on factors in addition to mere extension of foot area by the use of a fin. U.S. Pat. No. 2,727,668 to Cressi and Ferraro thus taught the use of a fin angled downwardly relative to the longitudinal axis of the foot. This angularity took into consideration the posture and leg movements of the human body while swimming and the direction of intended movement. To accommodate the angularity, a toe hole was provided.
In the early 1960s, Georges Beuchat of Marseille, France improved upon the Cressi-Ferraro fin structure by providing a two stage machine by the aid of vents in the medial portion of the fin part. The efficiency was improved by reducing drag, particularly during the power downstroke. The Beuchat JETFIN has been widely copied in recent years. Minor improvements have been suggested, such as equalizing the specific gravity of the fin, controlling the stiffness to achieve proper angularity of attack. The general objective has always been an increase in swimming efficiency and a corresponding decrease in fatigue.
The JETFIN, although it is a vastly improved machine as compared with the early single stage fins, nevertheless has certain imperfections. Some of these imperfections are believed to result from the fact that designers have considered only simple static force diagrams and angularity of musculoskeletal structures. In fact, a swim fin is a dynamic structure that moves through the water. One object of the present invention is to provide a swim fin so designed that the water flow is essentially laminar and free of excess turbulence.
Another imperfection is believed to result from the fact that the prior art fins assume different curvatures according to the direction of movement and the magnitude of the forces applied. As mentioned above, the control of proper curvature has been attempted by changing the composition of the material. In fact, it is angularity, not curvature that should be controlled, and angularity can be controlled by structural characteristics of bending, not merely by characteristics of materials. Another object of the present invention is to provide a swim fin in which angularity of the swim fin is accurately controlled both for the upstroke and for the downstroke whereby the ratio of power to fin area is markedly increased, which makes it possible to reduce the overall size of the swim fin without sacrificing total power.
It has been recognized that the swimmer has far less muscular power for the upstroke than for the downstroke. Since there is less power available, the machine operates better as a single stage propulsion device that as a two stage propulsion device. During this stroke, vents impede efficiency. Hence they are desirably closed during the upstroke. The prior art devices have achieved some measure of closing by virtue of a change in curvature. Another object of this invention is to provide a more effective closure for the upstroke.